1. Field of the Invention
This invention relates to a color converting apparatus and a color converting method for use with a printing machine when making color prints.
2. Description of the Related Art
In a printing system which carries out digital color printing, a different color definition is used for each of a display device such as a liquid crystal display for displaying images, a scanner for reading images, and a printing machine for performing printing. A color management for controlling printing colors of the printing machine is carried out by converting such different color definition for each digital device into a common color expression using ICC profiles. The ICC profiles are files of formats specified by the International Color Consortium, and are files defining characteristics of the color spaces of devices. These ICC profiles employ the CIE color system (CIEXYZ, CIELAB) as a common color display method.
A color printing machine performs printing, using the four color materials of cyan (C), magenta (M), yellow (Y) and black (K) which are generally called process colors. On the other hand, for colors difficult to express with color reproduction by the process colors, a specially color-mixed monochromatic ink or toner is prepared and is loaded in the printing machine as spot color (special color). A printing machine particularly suited for mass printing of a single article, such as package printing with a corporate color in large quantities, carries out color printing with a spot color designated as the corporate color, besides the process colors.
Incidentally, for POD (Print On Demand) printing which specializes in multi-article and small number of copies printing, since this prints only a required number of copies when required, a spot color cannot in many cases be loaded in the printing machine. Therefore, in POD printing, a spot color has to be reproduced by means of the process colors used in for printing.
When the printing ink colors used by the printing machine are the four colors of CMYK, for example, a spot color presented in RGB values on the display device will be expressed as separated into CMYK values in a printing color space. In conventional color reproduction of a spot color, the operator determines, for a solid portion of the spot color, CMYK values based on values obtained by measuring color samples or prints with a colorimetric machine. Other varied measures taken include a case of the operator selecting, from among color patches of a printed color chart, the color patch closest to the spot color displayed on the display device. A proposal has been made in Japanese Unexamined Patent Publication No. 2000-217007, according to which a color chart consisting of 27 color patches is printed, and the operator selects from the color chart the color patch closest to a spot color displayed on a display device, thereby realizing a color conversion with a high degree of approximation in color sense between the spot color displayed on the display device and the color outputted by a printing machine.
A further proposal has been made in Japanese Unexamined Patent Publication No. 2001-157074, according to which, when proof printing is carried out with a printing machine not loaded with a spot color for prior confirmation of the result of printed image in advance of printing by a printing machine loaded with the spot color, image data is generated for the proof printing by combining CMYK process colors and the spot color after color conversions are carried out with different conversion systems, respectively.
A further description will be made of a relationship between L*a*b* values in a device-independent L*a*b* color space, which are employed as conventional common color expression between devices, and CMYK values in a printing color space dependent on the colors of color materials of a printing machine. FIG. 9 is a graph showing a relationship between a monochromatic dot percentage and L*a*b* values. FIG. 10 is a graph showing hue variations in an a*−b* plane occurring with variations in the monochromatic dot percentage. FIGS. 9 and 10 show a plotting of each value when the dot percentage is changed at intervals of 10%.
The relationship between the dot percentage of single color C used as a process color and L*a*b* values is, as shown in FIG. 9, such that each value of L*, a* and b* has a tendency to diminish linearly and substantially equally with the dot percentage. Also the a*−b* plane presenting hue, as shown in FIG. 10, changes linearly and at a substantially constant rate with the variations in the dot percentage.
When reproducing the halftone of a spot color with the process colors, according to conventional practice, L*a*b* values of a solid of the spot color (with dot percentage at 100%) provided by a color material supplier such as an ink maker, or L*a*b* values obtained by measuring with a colorimetric machine a solid patch of the spot color printed with a printing machine, are first converted into coordinate values (CMYK values) in a CMYK color space which is a printing color space of the printing machine, using an ICC profile of a printing machine for output. Then, CMYK values of the spot color are calculated based on the CMYK values of the solid of the spot color (with dot percentage at 100%), on an assumption that the CMYK values of the spot color vary linearly with variations in the dot percentage (in a proportional relation therebetween). When, for example, the CMYK values of the solid of the spot color are CMYK=(50, 30, 20, 0), CMYK values of halftone with a dot percentage at 50% are calculated by multiplying the CMYK values of the solid by the 50% halftone dot percentage, which results in CMYK=(25, 15, 10, 0).
As shown in FIGS. 9 and 10, in the case of a single color, each value of L*, a* and b* and the hue vary linearly with variations in dot percentage. It is ideal that each value of L*, a* and b* of the spot color supplied as a monochromatic color material for a printing machine loadable with the spot color, and hue as well, will change similarly. Conventionally, therefore, even when a spot color is reproduced with the process colors, a technique (conventional technique) is conceivable which calculates halftone CMYK values by multiplying the CMYK values of a solid as they are by a dot percentage, based on an assumption that variations similar to those of a single color will occur with variations in dot percentage.
FIG. 11 is a graph showing a relationship between dot percentage and L*a*b* values of a spot color when the spot color is reproduced with the three colors of CMY by a conventional technique. FIG. 12 is a graph showing hue variations in an a*b* plane occurring with variations in the dot percentage when the spot color is reproduced with the three colors of CMY by the conventional technique. FIGS. 11 and 12 show an example of spot color which can be expressed by CMY=(100, 70, 10) when a solid of the spot color is separated into CMY, and show a plotting of each value occurring when the dot percentage is varied at intervals of 10%.
When halftone CMY (K) values are obtained from CMY=(100, 70, 10) with the dot percentage at 100% by the conventional method of calculation giving CMY=(50, 35, 5) if the dot percentage is 50%, each value of L* and b*, as shown in FIG. 11, can be said to vary linearly with variations in the dot percentage, but the value of a* shows a gentle arch form attaining a maximum at 50% dot percentage. In the a*−b* plane, as shown in FIG. 12, the directional property and rate of change with respect to the variations in the dot percentage are not uniform, and linearity is greatly impaired.
Thus, in printing with the CMYK values acquired by the conventional calculation technique, since the hue in the a*−b* plane varies significantly with variations in dot percentage, halftone portions can deviate from envisaged color sense even though color reproduction in a solid portion of a spot color is performed with high accuracy.